This invention is directed to a process for preparing melt stable polyarylates having a reduced viscosity of from about 0.4 to greater than 1.0 dl/g which comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid to produce a polyarylate having a reduced viscosity of from about 0.5 to greater than 1.0 dl/g and then adding a stabilizing amount of an aromatic phenol to the polyarylate to form a polyarylate substantially free of anhydride linkages.
Polyarylates are polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl)propane also identified as bisphenol-A, and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.
Many processes have been described in the literature for the preparation of polyarylate. One such process is the diacetate process. In the diacetate process, a dihydric phenol is converted to its diester derivative which is then reacted with aromatic dicarboxylic acid to form the polyarylate.
A proposed reaction mechanism for the preparation of polyarylates by the diacetate process is illustrated for the reaction of bisphenol-A diacetate and a mixture of isophthalic and terephthalic acids: ##STR1##
In addition to producing the polyarylate (E), the reaction of iso and/or terephthalic acids (B) and bisphenol-A diacetate (A) can produce anhydride (F), either by way of the intermediate mixed anhydride (C) or by the direct reaction of two molecules of the phthalic acid (B), accompanied by the loss of water. Due to the reactivity of anhydride (F), only a low equilibrium concentration of anhydride linkages will remain in the polymer backbone at the end of the reaction. These anhydride linkages are believed to be labile enough to cause loss of molecular weight when the polymer is exposed to high temperatures, i.e., 320.degree.-400.degree. C., which are those needed to fabricate the polyarylate. Therefore, elimination of these anhydride linkages prior to processing the polyarylate is deemed necessary to help prevent the loss of molecular weight.
It has been surprisingly discovered that the addition of an aromatic phenol at the end of the polyarylate forming reaction eliminates the anhydride linkages and stabilizes it without significantly effecting the aromatic ester linkage.
U.S. Pat. No. 4,011,196 describes the addition of a hindered phenol antioxidant or stabilizer in a small or stabilizing amount to a polyester resin-forming reaction mixture during the initial transesterification stage in which a zinc catalyst is used, or during the final polymerization stage before the desired intrinsic viscosity of the polymer is reached.
However, the transesterification process described in this reference would not produce anhydride linkages.
Japanese publication No. J5 0130-845 describes aromatic polyesters prepared from aromatic dicarboxylic acids and bispheols mixed with 0.01 to 1 weight percent of phenolic stabilizers. An example in this publication describes the addition of 0.5 parts of 2,6-di-tert-butyl-4-methylphenol dissolved in acetone to an aromatic polyester derived from 2,2-bis(4-hydroxyphenyl)propane and a 1:1 terephthalic and isophthalic acid dichloride mixture.
However, the interfacial polymerization of this reference would not be expected to produce anhydride linkages.